Texturing consists in creating a certain roughness at the surface of a substrate in order to enable multiple reflection of light incident on its surface, thereby leading to greater absorption of the light inside the substrate, i.e., to an increased light-confining effect. The roughness obtained in this way has two complementary effects: the first effect is to reduce the reflecting power or optical reflectivity of the surface; the second effect is to increase the length of the optical path traveled by the incident light inside the substrate. In a photocell, photovoltaic cell or solar cell, the increased light-confining effect gives rise to an increase in the effectiveness with which light is transformed into electricity.
Customarily, the roughness or texture of the surface of a substrate, in particular of a silicon or silicon alloy substrate or wafer may include randomized pyramids or a multitude of pits having a depth in the range of from 0.1 to 10 μm and a diameter in the range of from 0.1 to 10 μm.
Various methods have been proposed for texturing the surface of the said substrates.
Mention may be made of methods such as mechanical engraving, laser etching, photolithography, masking, sandblasting, mortizing, anodic oxidation, sputter etching, etc. However, these methods are complex and expensive to implement (cf. also the international patent application WO 01/47032 A1, page 2, line 26, to page 3, line 7).
The single crystal silicon substrates are mainly produced by sawing massive silicon ingots. However, crystal defects having a depth of several μm are caused by the sawing, which is known in the art as saw damage. Since these crystal defects are centres for the recombination of electron-hole pairs, it is necessary to remove them by way of a so-called saw damage etch.
The method of surface texturing presently employed on an industrial scale is characterized by the use of aqueous alkaline solutions based on sodium hydroxide or potassium hydroxide. These solutions have the property of etching silicon anisotropically depending on the crystallographic orientation of the grains situated at the surface, thereby modifying the surface morphology of the silicon. Etching speed is about 100 times greater on planes having the crystallographic orientation [100] than on [111] planes. This causes the surface to be textured in the form of randomized pyramids situated on [100] planes that trap the incident light, which effect is referred to as “micro-texturing”. However, for polycrystal silicon substrates, it is estimated that only 20% of the surface is constituted by grains having this crystallographic orientation. Therefore, the treatment is less effective with respect to optical reflectivity. This equally applies to silicon substrates manufactured by the edge defined film-fed growth (EFG) process or by the string ribbon process, which processes do not require sawing (cf. also the international patent application WO 01/47032 A1, page 1, line 22 to page 2, line 25). Thus, the treatment is fully effective only in the special case of single crystal silicon substrates having crystallographic orientation [100] at the surface to be etched.
In order to ameliorate these problems and drawbacks, aqueous acidic etching solutions and methods of their use have been developed in the past.
Thus, the American U.S. Pat. No. 5,949,123 or the European patent EP 0 773 590 B1 disclose an aqueous acidic etching solution containing hydrofluoric acid and nitric acid in a weight ratio of 1:3. This etching solution is used for creating pores in the silicon surface thereby creating a porous layer. In a second step, the porous layer is removed with an alkaline solution of sodium hydroxide, thereby creating the surface texture. Sulfuric acid is not used as a component.
The American U.S. Pat. No. 6,340,640 B1 or the German patent application DE 197 46 706 A1 disclose an acidic etching solution containing 12 parts by volume of 50% by weight of hydrofluoric acid, one part by volume of 69% by weight of nitric acid and 12 parts by volume of 85% by weight of phosphoric acid. Instead of the phosphoric acid, a carboxylic acid having a higher molecular weight than acetic acid can be used. Additionally, the acidic etching solution can also contain surfactants or ammonium fluoride. Sulfuric acid is not used as a component.
The international patent application WO 01/47032 A1 discloses acidic etching solutions containing 1 to 30% by weight of hydrofluoric acid, 5 to 30% by weight of nitric acid and 50 to 94% by weight of sulfuric acid and/or phosphoric acid. The preferred composition contains 10 to 16% by weight of hydrofluoric acid, 15 to 25% by weight of nitric acid, 15 to 25% by weight of sulfuric acid, 14 to 20% by weight of phosphoric acid and 20 to 30% by weight of water. An even more preferred composition contains 3 to 7% by weight of hydrofluoric acid, 3 to 7% by weight of nitric acid, 75 to 85% by weight of sulfuric acid and 5 to 15% by weight of water.
The Japanese patent application JP 2004-063744 discloses a two-step etching process, wherein an aqueous acidic etching solution A containing 36 to 42% by weight of hydrofluoric acid and 6 to 10% by weight of nitric acid and an aqueous acidic etching solution B containing 32 to 46% by weight of hydrofluoric acid and 2 to 6% by weight of nitric acid are used. Sulfuric acid is not used as a component.
The American U.S. Pat. No. 7,192,885 B2 or the international patent application WO 2004/100244 A1 disclose an aqueous acidic etching solution containing 10 to 40% by weight of concentrated hydrofluoric acid, 2 to 60% by weight of concentrated nitric acid and 20 to 55% by weight of water. Sulfuric acid is not used as a component.
The Japanese patent application JP 2005-311060 discloses an aqueous acidic etching solution containing 10 to 22% by weight of hydrofluoric acid, 15 to 31% by weight of nitric acid, the remainder being water. Sulfuric acid is not mentioned as a component.
The German patent application DE 10 2007 004 060 A1 discloses an aqueous acidic solution containing 0.8 to 2% by weight of hydrofluoric acid 15 to 40% by weight of nitric acid, 10 to 41% by weight of sulfuric acid, the remainder being 74.2% by weight or less of water. A preferred solution contains 1 to 1.7% by weight of hydrofluoric acid, 20 to 30% by weight of nitric acid, 18 to 35% by weight of sulfuric acid and 61% by weight or less of water. A particularly preferred solution contains 1.4% by weight of hydrofluoric acid, 27% by weight of nitric acid, 26% by weight of sulfuric acid, and 45.6% by weight of water. However, this solution is primarily used for the wet chemical edge isolation and, therefore, is used to solve a technical problem which is different from the problems of the texturing of surfaces. If it is used for this purpose, it exhibits only little etching power and leads to comparatively smooth surfaces.
Moreover, several commercial aqueous acidic etching solutions are available in the market.
For example, Spin-Etch™ BT contains 10 to 20% by weight of hydrofluoric acid, 20 to 30% by weight of nitric acid, 15 to 25% by weight of sulfuric acid and 10 to 20% by weight of phosphoric acid, the remainder being 35% by weight or less of water.
SI-Polish-Etch™ I contains 5 to 10% by weight of hydrofluoric acid, 36 to 38% by weight of nitric acid, 9 to 13% by weight of sulfuric acid and 16 to 20% by weight of phosphoric acid, the remainder being 34% by weight or less of water.
Spin-Etch™ D contains 1 to 7% by weight of hydrofluoric acid, 35 to 45% by weight of nitric acid, 10 to 20% by weight of sulfuric acid and 15 to 25% by weight of phosphoric acid, the remainder being 39% by weight or less of water.
Spin-Etch™ E contains 1 to 5% by weight of hydrofluoric acid, 5 to 10% by weight of nitric acid and 75 to 85% by weight of sulfuric acid, the remainder being 19% by weight or less of water.
Although these prior art aqueous acidic etching solutions are capable of ameliorating some of the problems and drawbacks associated with the aqueous alkaline etching solutions, they require a constant development to improve and balance their applicational property profile in order to meet the demands of the market. In particular, a more uniform color of the etched silicon substrates or wafers is required for aesthetic reasons, when the solar cells manufactured with these wafers are to be used in urban areas. Additionally, the grain boundary etching or defect etching, including the etching of crystal dislocation clusters, ought to be decreased in order to improve the stability of the silicon wafers and to lessen the risk of breakage of the silicon wafers and the solar cells, in particular during production. Last but not least, the efficiencies of the photovoltaic cells or solar cells needs constant improvement.